1. Technical Field
This invention relates to the field of methods for forming pressed paperboard products such as paper trays and plates and to the products formed by such processes.
2. Description of Related Art
Paperboard products, such as trays and plates, are formed, generally, by a method of molding fibers from a pulp slurry into the desired form of the product or by pressing a paperboard blank between forming dies into the desired shape. Manufacturing of paper products by the press forming process provides a number of advantages typically not found in products produced by other methods, such as the molded pulp process. Pressed paperboard products can be decorated and coated with a liquid-proof coating before being stamped by the forming dies into the desired shape whereas pulp molding processes permit the coating and decoration steps to occur only during or after the molding step. Coating and decorating a non-planar surface normally tends to be more expensive and difficult. Further, pressed paperboard products generally cost less, and require less storage space and can sometimes be transported in a collapsed condition, unlike molded pulp articles. Because press formed paperboard products can be manufactured more cost effectively than molded pulp articles and can be provided with features which improve product functionality and use, the press forming process is practiced by a substantial number of manufacturers in the paper product industry.
Although providing the advantages described above, the press forming method and products produced thereby present significant problems due to the resistance to bending and the change in area required in order to form the blank into the desired three dimensional shape. Press formed products for which round shapes are desirable, such as bowls, plates and trays, have been identified as a specific problem area.
In particular, the resistance to bending is a result of the paperboard stiffness. The stiffness level must be high enough to at least provide the finished product with the capability of supporting items, such as food, liquid or other objects, placed on or within the product without collapsing or excessively bending or twisting. A high stiffness level, while necessary to allow the product to withstand the tension and stresses experienced during use, presents problems during the forming operation, such as resistance to bending. As a further consequence to this resistance to bending, press formed paperboard products for which a round shape is desired have a tendency to assume a more nearly oval or egg shape, resulting in an irregularly shaped finished product.
The tendency of products for which a round shape is desired to assume an oval or egg shape is caused by the differing stiffness levels of the paperboard, and the resistance to bending caused thereby, in the different portions of the paperboard blank. The differing stiffness levels of the paperboard result from the differing memory or tendency of the paperhoard to spring back in the machine as compared with the cross machine direction of the paperboard. The machine direction of the paperboard relates to the direction in which the paper stock moved as it was being formed. Because this movement tends to cause the paper fibers to align differently in the machine and cross machine directions of the paper stock, many of the paper's properties, such as strength and elastic memory, will vary in the machine and cross machine direction. When a paperboard blank is forced to bend, the tendency to return to its original shape (i.e., flat) will normally be greatest in the machine direction and least in the cross machine direction. When a flat paperboard blank is press formed into a paper plate, a flat bottom wall and a curved upturned sidewall are normally formed. The tendency of the board, along the periphery of the flat bottom wall, to spring back to a flat shape is greatest in the machine direction and least in the cross machine direction. As a consequence, the side wall tends to be more nearly oval shaped than round shaped, which is clearly undesirable for a product for which a round shape is expected by the ultimate user. Accordingly, manufacturers of pressed paper products, and, in particular, manufacturers of pressed paper products for which a round shape or partial round shape is desired, would be in a better position to capitalize on their inherent lower manufacturing costs if paperboard blanks could be designed to more readily and consistently assume the desired three-dimensional product shape and if finished products could be formed to resist any inherent tendency to deviate from their pressed shape.
Many attempts have been made to overcome the deficiencies experienced during the formation and use of press formed paperboard products. A known method for enhancing the ability of paperboard blanks to be press formed into a uniformly shaped product, as well as to increase the product's ability to maintain its desired shape, is to provide score lines on the blank from Which the paperboard product is pressed. The provision of score lines causes pleats, seams, flutes or corrugations to form in a more nearly predictable and controlled fashion to accommodate surface area reduction at certain locations in the blank, as required to form the blank into the desired three dimensional shape.
Specifically, with respect to products for which a round shape is desired, the area reduction of the product's side wall and rim has been accommodated by providing a score line arrangement on the blank comprising a plurality of radially extending score lines circumferentially disposed around the periphery of the blank. Examples of this configuration may be seen in U.S. Pat. Nos. 4,606,496 and 4,721,499, both to Marx et al. and assigned to the applicant of the subject invention. Such an arrangement does, to an extent, reduce the blank's resistance to being press formed and control the formation of wrinkles. However, the score line arrangement of Marx et al. can only improve the formation process in the region where the score line arrangement is disposed (i.e., a portion of the side wall, the rim and the outer lip of the container). Where a finished product having a round shape is desired, a blank including the radially extending score line arrangement of Marx et al. would still present substantial resistance during the forming operation and, further, would tend to assume a more nearly oval or egg shaped finished product. This is due to the existence of other, non-scored regions of the product which are problematic and have not been accounted for in the Marx et al. design.
One region of a press formed paperboard product, particularly one for which a round shape is desired, that presents a substantial amount of resistance to bending and assuming the desired three dimensional curvilinear shape is the portion of the product in and near the periphery of the bottom wall of the product. The resistance to bending in this region is a result of the paperboard stiffness and, as is described above, the tendency of the paperboard, along the periphery of the flat bottom wall, to spring back to a flat shape is greatest in the machine direction and least in the cross machine direction. As a consequence, the side wall tends to be more nearly oval shaped than round shaped. The score line scheme of Marx et al. does not alleviate this problem and cannot because no score lines are formed in the region immediately beyond the periphery of the flat bottom wall.
Notwithstanding the known score line arrangements for improving press forming of paperboard products, there is a need for a configuration that can prevent the machine directionality of the paperboard from distorting the product from its desired shape (i.e., round), can reduce the blanks resistance to being press formed to the desired shape, and can improve the product's ability to maintain the desired shape (i.e., not return to its original shape), while providing a product with sufficient strength and rigidity to withstand the tension exerted on the product during normal use.